1. Field of the Invention
The invention relates to the purification of drinking water and is directed more particularly to an assembly for effecting purification at the point of use.
2. Description of the Prior Art
Attention has been focused on enhancing the safety of drinking water. Federal standards for bacteriological quality in drinking water have evolved steadily since the beginning of the last century, culminating in the Safe Water Drinking Act of 1974 and two significant amendments thereto, in 1986 and 1996. Over the years, domestic public water systems have improved the standards under which they operate, and water at the point of entry (POE) of distribution has generally been considered safe.
However, in recent years the public attitude toward the quality of the water supply at the POE has shifted. According to a 1999 survey by the Water Quality Association, one in five households was dissatisfied with the quality of their water supply. Better methods of pathogen detection, failing infrastructures in some communities, and concerns about the chemicals used in the treatment of water, are a few of the reasons. Drinking water quality is a special concern for Americans living in rural areas, where some waterborne pathogens are commonly found. Research has shown an abundance of Cryptosporidium and Giardia on dairy farms, both in the U.S. and abroad. Many rural communities do not have a large public water system and are vulnerable to waterborne pathogens.
Therefore, it is important to ensure the safety of drinking water from micro-organisms at the individual household levels, or point of use (POU). Events of Sep. 11, 2001, and the consequent “war on terrorism”, have further caused the shift in public thinking relative to the safety of Americans, both at home and abroad.
From the ground or surface water, delivery of drinking water to the consumer takes three main routes. First, municipalities undertake the task of purifying the water and supplying it to the consumers. Secondly, consumers use their own wells or other techniques for obtaining water. Lastly, commercial bottled water companies produce pure drinking water using either water from municipalities or water from other direct sources.
As a result of increased pathogen detection, and bio-terrorism threats, along with a general lack of funding for upgrading municipal water supplies, consumers are increasingly exploring the safe drinking water options. According to the 2001 Water Quality Association national survey, the percentage of U.S. population with home water treatment devices has increased from 27% in 1995 to 41% in 2001. It is estimated that more than ten million households will be looking for further enhancements of their drinking water for pathogen removal. The key providers in this market are water filtration companies using reverse osmosis or other nano-filtration technologies. However, there are many commercial water purification products in the market. Typically, they employ a combination of chemical, ultra violet (UV) radiation, ozonation, and filtration treatments in order to achieve maximum effectiveness, inasmuch as one method alone does not ensure protection against all micro-organisms.
Waterborne organisms of concern can be classified by their size (Table 1).
TABLE 1Relative Size of Waterborne Micro-OrganismsApproximate sizeMicro-organisms(in microns)ExamplesViruses0.002-0.5  Hepatitis, 0027 micronMeningitis, .2 micronBacteria, cocci0.5-1.5 Pseudomonas, 0.5-0.62 micron(spherical) andVibrio cholerae, 1 micronbacilli (rod-shaped)Protozoa2-15Giardia lamblia, 9-12 micronCryptosporidium, 4-6 micron
Most of the commercial and industrial devices for water purification in use today employ a combination of techniques classified in five main categories. Their application areas, principles of operation, effects on surviving microorganisms, and limitations of the processes are shown in Table 2.
TABLE 2Summary of Different Water Treatment ProcessesTreatmentDone atOperating PrincipleEffect on survivorsLimitationsUltravioletPOUmutagenInterference withLimited impact on hard-DNA replicationshelled micro-organismsChlorinationPOE/Production ofEvolution of resistantchlorination by-products,POUHypochlorous acidbacterial cells fromnot effective against C.that kills microbialthe fittest survivorsparvumpathogensThermalPOUdenaturation oflethal above certainRequires high input ofproteinstemperature (usuallyenergy to raise the65°)temperature to sufficientlevelFiltrationPOUSeparates the microbesNoneFlow rates, maintenancefrom the water basedon their sizesOzonationPOU/oxidation of cell walls,oxidationComplex generationPOEdamage to nucleic acidprocess, Need to eliminateresidual gases that may betoxic
It is clear that while all the techniques are effective to a certain extent, they have limitations in terms of operations, resistance of certain pathogens, and creation of toxic by-products. It is desirable to have a device that is effective against all kinds of microorganisms and will not generate toxic by-products.
Acoustic “cavitation” fills this need and is an ideal supplement to existing processes. Recent work has confirmed this by demonstrating that cavitation is effective in destroying E. coli and L. pneumophila. Cavitation is the phenomenon of creation in implosion of microscopic bubbles in a fluid and is caused by rapid changes in the fluid pressure. The primary mechanism of inducing the rapid change in pressure is transmission of ultrasound in the fluid. The sound is created either by a piezoelectric or magnetostrictive transducer. Coupling a sound transducer to the fluid medium and driving the transducer with sufficient power creates the alternating pressure cycle that induces cavitation.
The parameters that affect cavitation are input energy transmitted in the medium, frequency of the sound, number of sound pulses, and the ambient pressure of the fluid. Typically, the higher the frequency, the greater is the cavitation threshold and the smaller the bubble size There are three distinct stages associated with cavitation:                1. Nucleation—Cavitation bubbles are formed in this stage.        2. Bubble growth—The bubbles grow in this stage by absorbing energy from the alternating compression and expansion caused by ultrasound.        3. Implosion—In this stage the growing bubble reaches a critical size and implodes upon itself, because it cannot absorb any more energy.        
In the implosion stage, very high local temperatures and shearing forces are produced. This shear force and high temperatures are utilized in a number of applications designed for cleaning (acoustic cleaners) and homogenizing tissue samples (sonicators).
There is a need for a water cavitation assembly which is relatively simple and of inexpensive construction, easy to operate, and which can easily be installed in drinking water systems at points of use.
Further, in view of the continuing necessity for proper functioning of the purifying assembly, there is a need for a simple, easy to use, and inexpensive test apparatus and method, to facilitate quick and easy testing of the purifying assembly on a high frequency basis.